This invention relates to brushless DC motors electronically commutated and more particularly to brushless DC fan motors that are of simple construction, inexpensive to manufacture, and reliable.
A typical goal in the manufacture of fans is a motor that is very simple and consequently has a low manufacturing cost. In AC motors for fans, the side armature AC motor comes closest to achieving these goals. However, recently, DC motors for fans have become more and more attractive, particularly for fans used to cool electronics where DC power is available.
Brushless DC motors using Hall effect devices to sense the commutation points as the rotor rotates are well known in the art. One or more stator coils are repeatedly energized or have their energization reversed to effect relocation of the electromagnetic field produced by poles of the stator core. A permanent magnet rotor is continuously attracted to the new electromagnetic pole locations. For commutation, one or more Hall effect devices sense the location of the poles of the rotor permanent magnet to control the energization of the stator winding or windings, or a Hall device detects the position of one or more commutation magnets mounted to rotate with the rotor and provided especially to indicate, the changing the state of the Hall device, the commutation points as the rotor turns.
Many brushless DC motors have been complex in both their structure and their commutation circuitry. Where simple, low cost and reliable fan motors have been needed these brushless DC motors, which might more appropriately have been used for, say, precise disc or tape drives, have been too expensive for the simple purpose of fan rotation.
One type of simple DC brushless motor is described in U.S. patent application Ser. No. 416,504 filed Sept. 10, 1982, now abandoned, and assigned to the assignee of the present invention. The motor in the prior application comprises a rotor with an annular permanent magnet and a stator coil and electromagnet structure outside the annular magnet. The annular magnet of the rotor has oppositely polarized magnet segments alternately arranged about the rotor's circumference. The electromagnet structure includes pole pieces proximate the outer cylindrical surface of the rotor, and are magnetized by a coil with a single winding. A Hall effect device senses the passage of the rotor magnet segments to turn the single coil on and off. The location of the electromagnet pole pieces and of the Hall effect device are such that, each time the coil is energized, the correct polarities are established at the pole pieces to attract the next approaching segments or poles of the annular magnet. The coil is thus pulsed on when the polarities of the magnet segments approaching the poles are opposite to the poles, but is turned off when these polarities are the same as that of the poles. Therefore, for a rotor magnet having for example four magnet segments of each polarity, eight segments in all, spaced around the magnet, the coil will be pulsed on four times per magnet revolution. For a rotor magnet having equally sized segments of opposite polarity, the coil will be off for a total of one-half of the magnet revolution time.
In this prior motor design, because the rotor could stop in a "winding off" position, a permanent detent magnet is provided on the stator structure close to the periphery of the annular rotor magnet. This magnet magnetically detents the annular magnet so that the rotor is correctly positioned for start-ups. However, it has been found that the motor experiences vibration due to the interaction of the rotating magnet with the detent magnet and the pulsing winding, thus producing a torque ripple.